Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface showing a first image of the claimed design; and,

FIG. 2 is a second image thereof.

The outermost broken line rectangle depicts a display screen; the remaining broken lines depict portions of the graphical user interface; all broken lines form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 